This invention relates to flare light compensation in optical systems.
Flare light degrades an image formed by optical systems. Flare light is defined as non-image light which is imposed on the image formed by an optical system and is the result of the scattering and reflection of image light by the optical elements in the system. As a result of flare light, the dark portions of the image formed by an optical system appear lighter.
Optical systems are typically constructed from optical elements having anti-reflection coatings. These coatings reduce reflected light and so reduce flare. However, even in these optical systems, 1-3% of the light level found in the white areas of the image is reflected by the components in the optical system as flare light. This percentage is called the flare characteristic of the system.
Uniform flare light is caused by the scattering of light passing through the optical elements such that a fairly uniform light is imposed over all portions of the image. The intensity of uniform flare light is a function of the amount of light in the image. The brighter the image (that is, the more light areas) the greater the flare. The intensity of flare light, as described above, is also a function of how the components of the optical system scatter and reflect light. Finally, the intensity of flare light is a function of the wavelengths of the light present in the image. Taking these effects into account, uniform flare light intensity can then be described mathematically by the expression: EQU I.sub.f =f.intg..intg.I(x,y,.lambda.)dad .lambda./A
where f, a constant, is the flare characteristic related to the amount of white light in the image scattered by the optical elements as previously described, .lambda. is the wavelength of the light, da is the area over which the intensity is integrated, d.lambda. is the wavelengths over which the intensity is integrated and A is the total area of the image.
One of the effects of uniform flare light is the desaturation of the image color. This happens because the flare light, by its nature, may impose light of other colors upon the image. For example, if the brightest parts of the image are mostly red, the flare light produced will be mostly red and any portions of the image which are not red, for example green, will be degraded (desaturated) by the superposition of the red flare light on the green portion of the image.
Another result of flare light is that the dynamic range of a detector in a system is effectively decreased. To understand this decrease, consider a simple optical system composed of an object to be imaged, a lens, and a detector. When the detector is in absolute darkness, it produces a noise signal, called the dark current. Assume for the purpose of this example that the dark current signal is of magnitude 0.01 units. Assume further that when the detector views an object that is white and in strong light, such that it produces the largest signal of which it is capable, it produces a signal of magnitude 100 units. The dynamic range of the detector is then 10.sup.4 (that is, the detector is responsive to light in the range from 0.01 to 100 units). When the detector views a object that is absolutely black, ideally the signal should be equal to the dark current value. However, due to flare light caused by the scattering of light from areas adjacent to the black object, the black image formed is not absolutely black but is somewhat lighter. Assume that when this black image is viewed, the detector now generates a signal equal to 1 unit instead of the 0.01 units it would have generated had there not been flare light. The result is that the dynamic range of the detector has been decreased from 10.sup.4 to 10.sup.2 (that is, the detector is now responsive to light in the range from 1 unit to 100 units). Therefore, to utilize the greatest dynamic range to which the detector is capable of responding, it is necessary to cancel the effects of flare.
Previous methods of compensating for flare light have utilized the fact that the flare is dependent upon how the optical elements scatter light to compensate for flare light. These methods measured the amount of light scattered by the optical elements when there was no object imaged (i.e. an illuminated background) and compensated all subsequent images using this pre-measured flare value. However, since flare light is also function of the amount of light forming the image, the use of the same pre-measured value for all images resulted in some images being over-compensated and some being under-compensated, depending upon the how much light was present in the image being compensated.
The present invention compensates an image based upon how much uniform flare light is actually present and therefore reduces the tendency to under compensate or over compensate a given image.